This invention pertains to apparatus in general and a machine in particular for forming expanded resinous products, such as expanded polystyrene packing forms. More specifically, this invention pertains to such apparatus, adapted, by virtue of various interrelated design features and subassemblies, to produce expanded thermoplastic cellular products on an industrial scale in an efficient and reliable manner.
Conventional industrial equipment for making expanded resinous products, such as expanded cellular polystyrene packaging elements, generally comprise a single mold, into which partially pre-expanded resinous beads are introduced. This mold includes heating means for then heating the partially pre-expanded beads and causing an expansion thereof. Means are also provided thereafter for cooling the mold and the expanded product therein so as to interrupt the expansion step and cool the product for subsequent removal when the mold is opened. The inefficiencies in repeatedly heating and cooling such a mold are evident from, among other things, the typical cycle time in such machines. The cycle time for producing an individual molded product in each such machine is often on the order of 50 or more seconds. In addition, such machines must necessarily be relatively large and the molds relatively massive so as to withstand the repeated heating and cooling cycles while avoiding deformation and material failure.
In the literature and in some fairly recent machine developments, efforts are apparent to improve upon this conventional machine design. For example, U.S. Pat. No. 3,676,033-Buonaiuto and German Offenlegungsschrift No. 1 922 261 (believed to be equivalent to the Buonaiuto U.S. patent) disclose an apparatus wherein expanded cellular products are made in a two-step process, the partially formed and expanded product being tranferred from a first molding station to a second moding station by a frame element with the partially-formed product retained between planar members so as to confine the products during transfer. Separation of the heating and cooling steps in this operation is clearly disclosed. However, the second molding step in the Buonaiuto process involves some further forming of the expanded blank from the first mold. In addition, the molds must be open to remove the cooled, formed product from the second molding stage.
Though not published before the present invention, German Offenlegungsschrift No. 26 13 768 (published Oct. 13, 1977) discloses a two-stage apparatus for making expanded thermoplastic cellular products. This publication refers specifically to the above-referenced Buonaiuto patent and to German Pat. No. 20 11 928 discussed below.
Offenlegungsschrift No. 26 13 768 differs from Buonaiuto primarily in that it utilizes a second stage mold element, more specifically a non-heated, non-cooled second stage male mold as the transfer element. Thus, a performed expanded cellular blank is retained in a half of the heated first stage mold when the molds are open and the second stage mold transfer element is transferred up and into engagement with the open first stage mold, where it picks up the preform and then is moved back to be inserted into the second stage mold upon mold closure. Simultaneous forming of the preform in the heated first stage and end forming in the cooled second stage of the apparatus is specifically disclosed. Product removal from the open second stage mold occurs simultaneously with the transfer of the second stage mold transfer element to pick up the previously formed preform from the first stage mold. German Pat. No. 20 11 928 -Brandenburger also discloses a two stage apparatus for making plastic products. It, however, does not suggest application of this apparatus to making expanded thermoplastic cellular forms. It does, however, disclose the formation of a preform in the first stage of the apparatus and the reuse of a preform mold element as a transfer element by which the perform is moved into engagement with a second stage or end-forming mold. Draw forming of the preform in the end mold stage is accomplished by the application of vacuum on one side and air pressure on the other side to transform the preform from the preform mold transfer element into the shape of the end form mold. No comparable mating (male) mold half in the forming mold is used in this process.
The use of a mold element as a transfer member to subsequent molding stages is also seen in U.S. Pat. Nos. 3,129,464--Heider, 3,193,874--Jablonski, 3,342,913--Engel and 3,374,500--Drenning. Each of these patents pertain to the molding of foamed or cellular thermoplastic articles.
The use of mold elements as transfer members is also well known in other fields as shown, for example, by U.S. Pat. Nos. 3,054,141--Hammesfahr (rubber tire vulcanization), 3,407,433--Beebee et al (injection molding), 1,336,579--Oxley (salt block forming), and 2,003,702--Straub (concrete block making).
It should be noted that in each of the foregoing references, a forming or molding operation occurs in the stage after transfer. Thus the second stage mold or forming element in each case, where two stage molding is disclosed, involves a mold or shape of different configuration than in the article formed prior to transfer. Thus the article prior to transfer is better characterized in these disclosures as preforms. Of these, only Buonaiuto and Jegelka (Offenlegungsschrift No. 26 13 768) refer to the energy savings involved in the first stage heating and the second stage cooling of an expanded thermoplastic bead product.
To the extent the foregoing references in the expanded thermoplastic bead product field consistently indicate some forming or pressing operation in the second stage mold, it is apparent that the processes and apparatus disclosed would not be applicable for forming expanded thermoplastic cellular products of thicker cross-sections. It is in the forming of such products that significant problems and delay times are incurred to provide relative uniformity in cellular expansion and densification of the final product. To that end, various other teachings have been directed. For example, German Offenlegungsschrift No. 2 003 657 pertains to a procedure involving partial mold opening in conjunction with low pressure steam and subsequent re-closing of the mold to effect better expansion of the product into all mold corners and reducing the subsequent cooling time required. Similarly, British Pat. No. 923,189 discloses a procedure involving pressurized steam to permit more complete expansion of the product and British Pat. No. 945,073 discloses a staged heating process to effect uniformity in the expanded cellular article. The overall necessity of uniformly heating the expanded thermoplastic beads in these processes for forming thicker cross-section products necessarily involves relatively long delay times both in heating and in the subsequent cooling phase. Typically, such products are cooled either by very long delay times or by actual cooling of the mold or by spraying of the molded articles with cooling water or by combinations of these processes.
Applicants are aware that prior to their invention, others had developed a prototype machine for making expanded thermoplastic cellular products wherein a heated mold element was used as a transfer member and a separate second stage cooled mold, of essentially the same configuration as the first stage mold, was provided to permit cool down of the product simultaneous with the formation of a successive product, all for the purpose of reducing machine cycle time and energy consumption by the separation of the heating and cooling phases of the molding operation. In this machine, however, no provision was included for heating the hot mold transfer element. In addition, the hot mold transfer element operated effectively independently of the corresponding cold mold element which had to be withdrawn, also independently, to permit transfer of the hot mold element into the mating cold mold.
Having these various teachings in mind, it is the general objective of the present invention to provide more efficient apparatus capable of producing expanded thermoplastic cellular product, such as expanded polystyrene packaging elements of relatively thick cross-sections.
More specifically, it is the object of this invention to provide a machine inherently capable by its design of repeatedly producing expanded thermoplastic cellular product of high quality at high production rates and relatively low energy consumption.